So, I may take a really long road trip. My car says it should take 12 hours, but google maps says 15. I would need to do some stuff before crashing on the other end, so I could maybe do 12 but really couldn’t do 15 all in one day.
So, anyone have a good reason to believe one and not the other?
If it matters, my car is a Honda Pilot using the factory-provided DVD-based navigation system.
Thanks.
My car has an input for average highway speed, average road speed, and average backstreet speed. It seems to base travel time on those settings, rather than on any sort of database of street speeds.
By default, it had freeway speed as 75 or something. The speed limit is 60 here.
I routinely drive half-day and day long trips. The time set out in Google Maps is accurate for my driving.
Anecdote: Google Maps is more accurate for me than my aftermarket GPS. I drive about 200 miles a week.
If you keep your speed up (to the limit+a few mph) and your stops to an absolute minimum I think you can generally make a little better time than Google estimates…cutting 3 hours out of 15 sounds like quite a lot though, and of course if you run into traffic/weather/accident/construction/etc. issues all bets are off.
I usually drive the speed limit + 5 mph max over and find that Google Maps always greatly overestimates the time required. My GPS only slightly overestimates. I hate it because I will start out with an arrival time of, say, 6:34p.m. and as the miles tick off, I “gain” time. The arrival time keeps ticking down so after, say, 100 miles, I’m at 6:25p.m.
Then the family wants to stop for a restroom break and use up all of “my” time that I worked for. 
Back in the pre-GPS days we used AAA ‘TripTicks’, and they assumed an average speed of about 55 MPH to account for rest stops and traffic tie ups. You almost always got there quicker than that. Maybe Google Maps is doing the same thing.
Our Garmin usually gives a more realistic estimate.
Also, the early versions of MapQuest actually built in rest stops in a long trip. Say a 10 minute stop for every two hours of driving time. As a kid, my family would always make these stops. Lunch time? Let’s sit down for an hour and have a full meal; no grabbing a sub and eating on the road like I do when I travel.
So maybe Google tries to balance the two styles. As a kid, my family would be way past these estimates, and I am way under them. Maybe they do factor in stops.
We do a lot of travel for my kids sports. I always start with Google Maps but use my Garmin during the actual trip. Google is always way over. I drive the limit + 5-7 mph and we try to limit the stops to gas/pee/food all at the same time. We’ll eat the fast food at the restaurant, but won’t go to something where they wait on us (Cracker Barrel, Applebees, etc.). Just did an Indy to Myrtle Beach trip (790 miles) and Google said 13.5, others said around 13, and we did it in 12 (almost exactly what the Garmin said). Average speed was just over 65 mph and most of the way the limit is 70.